This invention relates to a method and apparatus for concentrating an aqueous solution, and also to a method and apparatus for obtaining heat energy which is higher or lower than that of the concentrated aqueous solution, by diluting the concentrated solution.
The method and apparatus for concentrating an aqueous solution in accordance with the present invention can also be utilized as a method and apparatus for separating the water contained in the aqueous solution therefrom.
The method and apparatus for concentrating an aqueous solution in accordance with the present invention can be used, for example, as a means or apparatus for concentrating an aqueous solution in a dilution type of heat pump, an absorption type of heat pump or an absorption type of refrigerator.
Heat is generated when water is added to aqueous solutions of certain acids such as sulfuric or nitric acid. A process of obtaining high-temperature heat energy from a low-temperature heat source by utilizing this property is known.
The heat generated by the dilution of an aqueous solution of sulfuric or nitric acid increases greatly with increased concentration of the sulfuric or nitric acid. It is therefore necessary to dilute a high-concentration aqueous solution in order to obtain high-temperature heat energy, and hence a method of obtaining a high-concentration aqueous solution is important.
A boiling evaporation method is known as one method of obtaining a highly concentrated solution by concentrating an aqueous solution. Such a boiling evaporation method and a process of generating high-temperature heat energy using this method are described in, for example, the magazine "Thermal Power Generation", Vol. 28, No. 11, Nov. 1977, pp. 1091-1096.
This reference discloses a circulation process in which an aqueous solution containing nitric acid is effected by heating the solution above its boiling point and concentrated by evaporation of a contained water, the concentrated solution is diluted, and the diluted aqueous solution is again evaporated and concentrated. The concentration of the aqueous solution containing the nitric acid is effected by heating the solution to a temperature above its boiling point in a reduced-pressure vessel kept at about 9 mmHg, so as to evaporate and remove the contained water.
This boiling evaporation method needs a heat source for heating the aqueous solution to a temperature above its boiling point, and a vacuum apparatus. Furthermore, a gas-liquid separator is also necessary because part of the aqueous solution is accompanied by the vapor and scattered.
Therefore there is a strong demand to develop a method and an apparatus which can concentrate an aqueous solution to a high concentration without resorting to a boiling evaporation method, such as one that can accomplish a high concentration containing a solute at a concentration of at least 30% from a low-concentration solution, or one that can further concentrate the high-concentrated solution to a higher concentration.
The provision of a concentrated solution of a concentration of at least 30% has technical significance when generating high- or low-temperature heat energy in a subsequent dilution step. Unless the solution is concentrated to a concentration of at least 30%, high- or low-temperature heat energy can not be obtained.
The method of concentrating an aqueous solution utilizing the boiling evaporation method has the following problems.
In a boiling evaporation method, the solution is heated to boiling point (the temperature at which the partial vapor pressure of the solution coincides with atmospheric pressure), and/or the atmospheric pressure is reduced to the partial vapor pressure of the solution in order to induce evaporation throughout the solution, and thus increase the evaporation area.
The solution must also be well stirred, and heat must be transferred rapidly in order to induce the evaporation (boiling) throughout the solution. When the solution is stirred, the temperature of a heating surface must be 20.degree. to 80.degree. C. (degrees of superheat) higher than the boiling point, so that large quantities of bubbles are generated from the heating surface, the solution must be well stirred by those bubbles, and evaporation must be generated on the bubble surfaces within the solution.
When an 80% aqueous sulfuric acid (H.sub.2 SO.sub.4) solution is concentrated, for example, its boiling point is about 200.degree. C. at atmospheric pressure, and hence a heat source providing at least from 220.degree. to 280.degree. C. is necessary to enable that degree of superheat. Since a 60% aqueous lithium bromide (LiBr) solution has a boiling point of 155.degree. C., a heat source providing at least 175.degree. to 235.degree. C. is necessary to enable that degree of superheat in order to concentrate the solution by boiling evaporation at atmospheric pressure.
On the other hand, in accordance with methods using vacuum, boiling evaporation is effected at an absolute pressure of between about 50 and about 100 mmHg, from considerations of the vacuum-holding performance of a practical apparatus involving heating. When, for example, boiling evaporation is effected at an absolute pressure of 50 mmHg, the concentration unit must be sealed and kept under vacuum at between 150.degree. and 210.degree. C. for an 80% aqueous sulfuric acid (H.sub.2 SO.sub.4) solution and between 105.degree. and 165.degree. C. for a 60% aqueous lithium bromide (LiBr) solution, to enable the required degree of superheat, because the temperature drop slightly.
In accordance with the boiling evaporation method, further, the aqueous solution is likely to be picked up by the resultant vapor and be lost, so that a gas-liquid separator such as a cyclone or demister must be provided. In addition, not only sulfuric acid, but also lithium bromide, becomes more corrosive with increasing temperature, so that countermeasures must also be taken to prevent corrosion of the inner surfaces of the apparatus.
The use of a hydrophobic porous membrane, through which vapor can permeate but not liquid, is known when separating water from an extremely low-concentration solution such as in a method of distilling brine. (U.S. Pat. No. 3,340,186).
This prior art reference discloses a method wherein vapor is generated from brine, this is passed through a hydrophobic porous membrane, and is thereafter cooled and condensed. The method of this kind is directed to brine solutions of concentrations of as low as 3 to 4% for the following reasons.
It is known that as salt concentration increases, the resultant rise in the boiling point is generally rapid. With surface evaporation which does not involve boiling, evaporation occurs in only an extremely thin surface layer because liquid mixing is less, and a high-concentration layer occurs locally in the surface layer (this is the concentration polarization phenomenon). This phenomenon varies according to the solute, and the rate of evaporation, etc., but a surface layer of a concentration which is several percent higher than the bulk concentration (average concentration) is formed.
When the concentration of the solution is low, the rise in the boiling point due to the concentration polarization is not very great; but when the concentration is high, the rise in the boiling point is large and evaporation from the surface layer is greatly impeded.
In an aqueous LiBr solution of a concentration of 10% (boiling point: 102.degree. C. (point a)), for example, the boiling point rises by only 1.degree. C., i.e. to 103.degree. C. (point b), even when the surface layer is concentrated by 5% to 15% by concentration polarization, as shown in FIG. 4; but when a concentration of 60% (boiling point 155.degree. C. (point c)) is concentrated by 5% to 65%, the boiling point rises by as much as 15.degree. C., i.e. to 170.degree. C. (point d), so that evaporation from the surface is greatly impeded.
For the reasons described above, a boiling evaporation method has been employed in the past for concentrating a high-concentration solution, because liquid mixing is less in a non-boiling evaporation method and thus concentration polarization takes place. Since the hydrophobic porons membrane method also belongs to the class of non-boiling evaporation methods, it has been primarily directed to the concentration of low-concentration solutions.
Other methods using membranes include a reverse-osmosis membrane method in which a semi-permeable membrane such as a cellulose acetate membrane is used, and an electrophoretic method in which an ion exchange membrane is used. However, these methods are not suitable for concentrating a high-concentration solution.
In the reverse-osmosis membrane method, osmotic pressure increases rapidly with an increase in concentration, and the pressurizing force necessary to provide membrane permeation becomes several hundred of kg/cm.sup.2, and the concentration enabled by the membrane separation in practice is only a few percent.
In the electrophoretic method, ions in the solution are separated by electrophoresis, but the ion mobility and separability drop rapidly if the salt concentration increases. Moreover, the electrical conductivity of the solution increases and current flows more easily, so that the current efficiency drops, and the concentration that can be separated in practice is also only a few percent.
Therefore these two methods are used in practice solely for desalination, etc. Even if an attempt is made to apply these methods to the concentration of a solution to a high-concentration, they can be employed only auxiliarily in combination with evaporation concentration.